This research proposal is composed of three interrelated projects: 1) the ionic mechanism of the plateau of the action potential in frog ventricle using-gap voltage clamp technique. This study is being carried out in three phases: a) development and perfection of guard-gap voltage clamp technique, making measurement of true membrane current in multi-cellular tissues possible; b) the nature of rectifying characteristics of myocardial cell during the plateau; c) finally, separation of the ionic components of the plateau currents. 2) The electromechanical and electrophysiological characteristics of uni- layered heart obtained from tunicate, Boltinia ovifera. The perfusion of this myocardial tube and its voltage clamping accompanied with tension measurements will be undertaken. A similar technique to that employed on the giant axon of squid will be employed. The ionic nature and a possible carrier-mediated transport of Ca plus 2 and K plus will be studied in this uni-layered heart without inherent complications caused by extracellular unstirred layers or inhomogeneity in clamped potential due to anamalous distrubution of current. 3) The mechanisms underlying the change in the duration of the action potential in the hearts of fetal, neonatal and adult mammals. This phase of the project seeks to understand the marked variations in the duration of the plateau occurring with growth. Electronmicroscopic and electrophysiological techniques will be used to correlate the functional variation with ultra-structural development.